Separator for alkali-zinc battery

ABSTRACT

A separator for alkali-zinc battery comprising a microporous membrane having alkali resistance, a part of the porous membrane having highly persistent hydrophillic property and the remaining part having water repellent property. In the part having hydrophillic property, precipitation of ZnO can be controlled sufficiently because of its large persistency of hydrophilic property so that dendrite short-circuiting in the battery can be prevented satisfactorily. In the part having water repellent property, O 2  gas is permeable well through it so that a decrease in capacity of battery can be avoided.

This application is a Rule 60 continuation of application Ser. No.07/920,288 filed Aug. 13, 1992 now U.S. Pat. No. 5,320,916.

TECHNICAL FIELD

This invention relates to a separator for alkali-zinc battery for use ina portable equipment power source, a portable power source and anelectric vehicle power source etc.

BACKGROUND ART

The alkali-zinc battery has a high energy density and a high powercharacteristic. Because of a high solubility of zinc, however, zinc acidions existing in electrolyte at the time of charging have deposited onan anode in dendrite form or spongy form, a short-circuiting piercingthrough the separator has taken place, or a shape change has occurred tominimize a utilization factor; so that a cycle life of the battery hasbeen short.

A micro-porous membrane comprising polypropylene, polyethylene etc.subjected only to surface active agent treatment has been used for aconventional separator for alkali-zinc battery. Such a separator has adesirable property for use in a sealed-type battery in which O₂ gas isabsorbed by a zinc electrode, because O₂ gas is permeable through it.However, since it is subjected only to the surface active agenttreatment, its hydrophilic property is weakened while it is immersed inelectrolyte. Further, a mechanism wherein ZnO is gradually precipitatedin micro-pores to bring about short-circuiting, i.e. a dendriteshort-circuiting, can not be avoided sufficiently so that it is notsuitable for an application requiring a long-term storage or acharge/discharge cycle life of a long period.

In order to improve the above disadvantages, there has been used aseparator fabricated by a semi-permeable membrane and a micro-porousmembrane placed one upon another. However, the permeability of O₂ gascan not be expected in these membranes.

DISCLOSURE OF THE INVENTION

An object of this invention is to provide a separator for alkali-zincbattery, which has both properties of a semi-permeable membrane and amicro-porous membrane so that a dendrite short-circuiting can be avoidedsufficiently and a decease in battery capacity can be controlled bymaking O₂ gas to be permeable well.

This invention provides a separator for alkali-zinc battery comprising amicro-porous membrane having alkali resistance, a part of the porousmembrane having a high hydrophilic property and the remainder having awater repellent property.

According to this invention, the precipitation of ZnO is sufficientlycontrolled in the part having the hydrophilic property because thishydrophilic property is highly durable. Consequently, the dendriteshort-circuiting in battery can be avoided sufficiently. In the parthaving the water repellent property, O₂ gas is highly permeable.Consequently, the decrease in battery capacity can be controlled.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical sectional partial view showing a separator ofembodiment 1 of this invention.

FIG. 2 is a diagram showing cycle life characteristics of batteriesusing separators of embodiments 1, 2 and 3 of this invention and abattery using a conventional separator.

FIG. 3 is a diagram showing a change in charge/discharge cycle number incase when a ratio of pores filled with ion permeable resin to the entirepores is changed.

FIG. 4 is a vertical sectional partial view showing the separator of theembodiment 2 of this invention.

FIG. 5 is a view viewed in a direction of arrow V of FIG. 4.

FIG. 6 is a plan view showing another example of the embodiment 2.

FIG. 7 is a plan view showing a further another example of theembodiment 2.

FIG. 8 is a vertical sectional partial view showing the separator of theembodiment 3 of this invention.

FIG. 9 is a vertical sectional partial view showing another example ofthe separator of the embodiment 3.

FIG. 10 is a vertical sectional view showing a further another separatorof the embodiment 3 of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1)

FIG. 1 is the vertical sectional view showing the separator of theembodiment 1 of this invention. In FIG. 1, 1 denotes a micro-porousmembrane, and a membrane having a trade name of "CELGARD #3401" (made byDAICEL CHEMICAL INDUSTRIES Ltd.) is used therefor in this figure. Thisis a porous membrane made of poly-propylene having a thickness of 25microns, a porocity of 38%, and pore diameters of 0.05 through 0.125microns. This porous membrane is subjected to surface active agenttreatment. 2 denotes a cellulose which is an ion permeable resin filledin about 90% pores of the porous membrane 1. 1a denotes pores in whichthe cellulose is filled, and 1b denotes about 10% remaining pores inwhich the cellulose is not filled. The pores 1b are uniformlydistributed over a surface fronting on a plate surface. The pore 1b isprovided with water repellent property by removing the surface activeagent. In order to fill the cellulose 2, viscose is applied on an about90% area of surface of the porous membrane 1 and impregnated underreduced pressure to be solidified. As described above, the pores 1a arefilled with the cellulose and the pores 1b are provided with the waterrepellent property, so that a separator 10 is composed.

A property of the separator 10 thus constructed was investigated asfollows. A battery A using the separator 10, a battery B using aseparator comprising a simple micro-porous membrane made ofpolypropylene, and a battery C using a separator comprising a simplemicro-porous membrane made of polypropylene and a cellophane forming asemi-permeable membrane, were prepared each two cells, respectively.Electrodes, containers and electrolyte composing the batteries were thesame for all of them. Namely, a zinc electrode forming an anode wasfabricated in such a manner that a punched copper current collector onwhich a number of pores were made was used as a core metal, and zincactive material sheets were pressure crimped on both faces of the coremetal. A nickel electrode forming a cathode was fabricated in such amanner that an active material having a principal component of nickelhydroxide was filled in a sintered nickel plaque by means of thechemical impregnation method. Non woven fabric made of polypropylene wasused for the container. The electrolyte was KOH solution having aspecific gravity of 1.35. A nominal capacity of battery was 10 Ah.

Changes in capacities of the above batteries A, B and C wereinvestigated by repeating charge and discharge operations under thefollowing conditions. The discharge was conducted at 2A until a voltageper cell reached 1V. The charge was conducted at 1A up to a chargecapacity corresponding to 105% of a discharge capacity. Open circuitvoltages of the above batteries A, B and C were 1.70 V, voltages at 50%discharge were 1.65 V. Results are shown in FIG. 2.

As seen from FIG. 2, a decrease in discharge capacity is small in thebattery A even when the cycle is repeated. In the battery B, the gasabsorption is executed well and the decrease in capacity isapproximately equal to that of the battery A because it consists of thepolypropylene micro-porous membrane. However, the capacity abruptlydecreases caused by the short-circuiting due to the dendrite of zincbecause it is porous. In the battery C, the gas absorption is hard to beexecuted to cause the decrease in capacity because the cellophane isused.

In case of the separator 10 as described above, the cellulose is filledin the pores 1a, so that it has an excellent persistency in ionpermeability as compared with a separator subjected only to the surfaceactive agent treatment, ZnO is prevented from precipitating in the pores1a so as to avoid the dendrite short-circuiting sufficiently. Since thepores 1b are provided with the water repellent property, O₂ gas is wellpermeable so as to control the decrease in battery capacity. Further,the pores 1b are uniformly distributed over the surface fronting on theplate surface, so that O2 gas is absorbed uniformly and a change inshape of anode is minimized. Accordingly, a battery having an excellentcycle life can be provided by the separator 10.

The percentage of the pores 1a filled with cellulose to the total poresis about 90% in the separator 10 used for the battery A, however, thispercentage is not limited to this value. FIG. 3 is the diagram showingthe change in charge/discharge cycle number in case when a ratio ofpores 1a to the total pores in separator 10 is changed. The axis ofabscissa denotes the percentage of pores 1a and the axis of ordinatedenotes the charge/discharge cycle number. According to this diagram,even when the percentage of pores 1a is extremely small, the cyclenumber increases if any pores filled with cellulose exist. However, inthe event when the pores 1a occupy approximately the entire part i.e.the percentage exceeds a point X (about 98%), the cycle life decreasesbecause O₂ gas absorption becomes insufficient due to an excessivelysmall quantity of pore 1b. Consequently, it is enough to set thepercentage of pores 1a to a value larger than zero and smaller than thepoint X, so that the percentage is not limited to about 90%. However,the percentage is preferably set to about 90% because the peak of cyclelife exists at about 90%.

In place of the membrane made of polypropylene, those made ofpolyethylene and nylon etc. may be used for the microporous membrane.Further, in place of cellulose, poval (polyvinyl alcohol) etc. may beused for the ion permeable resin.

As describe above, the present embodiment is able to provide theseparator for alkali-zinc battery which can prevent the dendriteshort-circuiting satisfactorily and can control the decrease in batterycapacity by allowing O₂ gas to be permeable well. Therefore, theseparator of this embodiment is able to provide the alkali-zinc batteryhaving an excellent charge/discharge cycle life so that its industrialvalue is extremely large.

(Embodiment 2)

FIG. 4 is the vertical sectional partial view showing the separator ofthe embodiment 2 of this invention. FIG. 5 is the view viewed in thedirection of arrow V of FIG. 4. In the figures, 11 denotes a first layerforming the micro-porous membrane composed of two sheets placed one uponanother having trade name of "CELGARD #3401" (made by DAICEL CHEMICALINDUSTRIES Ltd.). The "CELGUARD #3401" is similar to that of theembodiment 1. A thickness of the first layer 11 is 50 microns. The firstlayer 11 is subjected to the surface active agent treatment. 12 denotesa cellulose forming an ion permeable resin coated on the entire surfaceof the first layer 11. Although not shown in the figure, micro-pores ofthe first layer 11 are filled with the cellulose 12. Viscose is appliedon the entire surface of the first layer 11 and impregnated underreduced pressure so as to be solidified, and the cellulose 12 can thusbe applied. 13 denotes a through hole having a circular section formedon the first layer 11. Many through holes 13 are made on the layer.

14 denotes a second layer forming a micro-porous membrane, and a sheethaving trade name of "HIPORE 2100" (made by ASAHI CHEMICAL INDUSTRY Co.,Ltd.) is used therefor. The "HIPORE 2100" is a porous membrane made ofpolyethylene. A thickness of the second layer 14 is 100 microns. Thesecond layer 14 is not subjected to the surface active agent treatment.Namely, the second layer 14 has water repellent property. The secondlayer 14 has a flat square shape, and is stuck at portion 14a by meansof melting adhesion with heat or ultra-sonic wave to a portion 11a of asurface of the first layer 11 so as to cover the through hole 13. Anarea of one second layer 14 is set to under 20 mm² incl., and a secondlayer 14 having a side length of 4 mm is used in this case. A diameterof the through hole 13 is set to 3 mm. One second layer 14 covers onethrough hole 13, and the entire second layer 14 is uniformly formed anddistributed to occupy an area of under 20% incl. of a surface frontingon the plate surface. Namely, the through hole 13 is previously formedin consideration that the second layer 14 can be distributed in theabove-mentioned manner.

A property of the separator 20 thus constructed was investigated in thesame way as the embodiment 1. That is, a battery A constructed in thesame way as the embodiment 1 using the separator 20, and a battery B andBattery C similar to those used in the embodiment 1 were prepared,charge/discharge operations were repeated in the same conditions as theembodiment 1, thus the changes in capacity was investigated. The resultswere the same as FIG. 2. Namely, the decrease in capacity was small incase of the battery A even when the cycle was repeated.

As described above, since the cellulose 12 (ion permeable resin) isapplied over the entire surface of the first layer 11 in case of theseparator 20, its persistency of ion permeability is excellent ascompared with that subjected only to the surface active agent treatment.Accordingly, in a part of the first layer 11 to which the second layer14 is not stuck, the precipitation of ZnO is controlled so that thedendrite short-circuiting in battery can be avoided satisfactorily. Onthe other hand, the second layer 14 has the water repellent propertybecause it is not subjected to the surface active agent treatment. Forthis reason, in a part to which the second layer 14 is stuck, O₂ gas iswell permeable through the second layer 14 and the through holes 13 sothat the decrease in capacity of battery can be controlled. In addition,O₂ gas is absorbed uniformly and the shape change of anode is minimizedbecause the second layer 14 is uniformly distributed on the surfacefronting on the plate surface. Accordingly, a battery having anexcellent cycle life can be provided by using the separator 20. In theevent when an area of one second layer 14 is larger than 20 mm² and/orwhen a percentage of an area occupied by the entire second layer 14 tothat fronting on the plate surface is larger than 20%, an effective areaof the zinc electrode is minimized so that these cases are notpreferable.

The second layer 14 is provided only on one surface in the separator 20thus constructed, however, it may be provided on both surface.

The through hole covered by one second layer 14 is formed into onecircular sectional shape in the separator 20 thus constructed, however,it is not limited to that shape. The through hole may be formed intoplural holes 13a having circular sectional shapes as shown by FIG. 6 orinto plural holes 13b having slit shapes as shown by FIG. 7. Naturally,it is not necessarily limited to the circular sectional shape but may bepolygonal shapes.

In place of the membrane made of polypropylene, those made ofpolyethylene and nylon etc. may be utilized as the micro-porous membraneused for the first layer 11. In place of the membrane made ofpolyethylene, those made of polypropylene and nylon etc. may be utilizedas the micro-porous membrane used for the second layer 14. Further, inplace of cellulose, poval may used for the ion permeable resin.

As describe above, the present embodiment is able to provide theseparator for alkali-zinc battery which can prevent the dendriteshort-circuiting satisfactorily and can control the decrease in batterycapacity by allowing O₂ gas to be permeable well. Therefore, theseparator of this embodiment can provide the alkali-zinc battery havingan excellent charge/discharge cycle life so that its industrial value isextremely large.

(Embodiment 3)

FIG. 8 is the vertical sectional partial view showing the separator ofembodiment 3 of this invention. The separator of this embodiment isformed by using a micro-porous membrane having trade name of "CELGARD#2400" (made by DAISEL Chemical Co.). The "CELGARD #2400" is a porousmembrane made of polypropylene and not subjected to the surface activeagent treatment, and has a thickness of 25 microns, a porosity of 38%and a max. pore diameter of 0.05×0.125 microns. In FIG. 8, 21 denotes ahydrophilic portion and 22 denotes a water repellent portion, and theseparator of this embodiment consists of the two portions 21 and 22. Thehydrophilic portion 21 is uniformly distributed so as to occupy about90% area of a surface of the above porous membrane i.e. a surfacefronting on the plate surface, and the water repellent portion 22 alsoconsists of plural banded portions 22a uniformly distributedcorresponding to the hydrophilic portion 21. The percentage of areaoccupied by the hydrophilic portion 21 is not limited to about 90% sofar as the percentage of area occupied by the water repellent portion 22is under 20% incl. An area of one banded portion 22a of the waterrepellent portion 22 is set to under 20 mm² incl.

The hydrophilic portion 21 is formed by radiation graft polymerizing amonomer acrylic acid after being subjected to heat treatment, so that itbecomes thinner than an original membrane thickness X due to the heattreatment. Pores of the hydrophilic portion 21 are filled up due to theheat treatment, and its hydrophilic property is based on a hydrophilicgroup of acrylic acid. Monomer used may be any monomer so far as it hasa hydrophilic group, and may be methacrylic or styrene sulfonic acid.

The water repellent portion 22 is formed by leaving the above porousmembrane as it is. Namely, it is formed by creating the hydrophilicportion 21 on the above porous membrane surface under a state where alead mask covering an area of the water repellent portion 22 is attachedto the surface. Micro pores are left on the water repellent portion 22as they are. A shape of the water repellent portion 22 is determined bya shape of portion covered by the mask, and the shape may be circular,polygonal or slit for instance.

A property of the separator 30 thus constructed was investigated in thesame way as the embodiment 1. That is, a battery A constructed in thesame way as the embodiment 1 using the separator 30, and a battery B andBattery C similar to those used in the embodiment 1 were prepared,charge/discharge operations were repeated in the same conditions as theembodiment 1, thus the changes in capacity were investigated. Theresults were the same as FIG. 2. Namely, the decrease in capacity wassmall in case of the battery A even when the cycle was repeated.

As described above, since the separator 30 includes the hydrophilicportion 21 formed by radiation graft polymerizing the monomer acrylicacid, the persistency of ion permeability is excellent as compared withthat subjected only to the surface active agent treatment. Accordingly,the precipitation of ZnO is controlled in a part of the hydrophilicportion 21. Further, since micro-pores of the hydrophilic portion 21 arecompletely filled up by the heat treatment, the precipitation of ZnO canbe avoided positively. Therefore, the dendrite short-circuiting inbattery can be avoided satisfactorily. Moreover, the separator 30includes the water repellent portion 22, and O₂ gas is well permeablethrough the water repellent portion 22. For this reason, the decrease incapacity of battery can be controlled. In addition, O₂ gas is absorbeduniformly and the shape change of anode is minimized because the waterrepellent portion 22 is uniformly distributed on the surface fronting onthe plate surface. Accordingly, a battery having an excellent cycle lifecan be provided by using the separator 30. In the event when an area ofone banded portion 22a of the water repellent portion 22 is larger than20 mm² and/or when a percentage of an area occupied by the waterrepellent portion 22 to that fronting on the plate surface is largerthan 20%, an effective area of the zinc electrode is minimized so thatthese cases are not preferable.

A micro-porous membrane made of polyethylene or nylon may be used inplace of the micro-porous membrane made of polypropylene.

The entire part of the above hydrophilic portion 21 of the separator 30thus constructed is subjected to the heat treatment, however, the heattreatment is completely negligible as shown by FIG. 9 or some part of itmay be subjected to the heat treatment as shown by FIG. 10. In FIG. 9, athickness of the hydrophilic portion 21 is not reduced as compared withan original thickness X. And in FIG. 10, the hydrophilic portion 21 iscomposed of a portion 21b formed by radiation graft polymerizing themonomer acrylic acid after being subjected to the heat treatment, and aportion 21c formed by radiation graft polymerizing the monomer acrylicacid without being subjected to the heat treatment. The portion 21b isformed on a boundary between the hydrophilic portion 21 and the waterrepellent portion 22. The portion 21c is not subjected to the heattreatment so that it maintains the original thickness X. According tothe example of FIG. 10, occurrence of the dendrite short-circuiting onthe boundary between the hydrophilic portion 21 and the water repellentportion 22 can be controlled as compared with that not subjected to theheat treatment entirely.

As describe above, the present embodiment is able to provide theseparator for alkali-zinc battery which can prevent the dendriteshort-circuiting satisfactorily and can control the decrease in batterycapacity by allowing O₂ gas to be permeable well. Therefore, theseparator of this embodiment can provide the alkali-zinc battery havingan excellent charge/discharge cycle life so that its industrial value isextremely large.

Industrial Applicability

The separator of this embodiment is able to provide the alkali-zincbattery having an excellent charge/discharge cycle life so that itsindustrial value is extremely large.

What is claimed is:
 1. A separator for a sealed alkali-zinc battery inwhich a cathode is isolated from a zinc anode by a separator and O₂ gasgenerated at the cathode is absorbed comprising a micro-porous membranebeing O₂ gas permeable to an extent sufficient to permeate O₂ gas fromthe cathode to the zinc anode, said membrane having a plurality of poresand alkali resistance, wherein a part of said porous membrane ishydrophilic due to radiation graft polymerizing of a monomer having ahydrophilic group, and the remaining part being hydrophobic.
 2. Aseparator for an alkali-zinc battery as set forth in claim 1, in whichpores at the hydrophilic part are closed due to previous heat treatment.3. A separator for an alkali-zinc battery as set forth in claim 2, inwhich said heat treated part is disposed at least on a boundary betweensaid hydrophilic part and said hydrophobic part.
 4. A separator for analkali-zinc battery as set forth in claim 1, in which approximately allof the hydrophobic part is composed of uniformly distributed pluralbanded portions, one banded portion having an area of under 20 mm², andthe hydrophobic part occupies an area of under 20% of a surface frontingon a plate surface.
 5. A separator for a alkali-zinc battery as setforth in claim 4, in which each of said banded portions has a circular,polygonal or slit shape.
 6. A separator for a alkali-zinc battery as setforth in claim 1, in which said monomer is acrylic acid, methacrylicacid or styrene sulfonic acid.
 7. A separator for a alkali-zinc batteryas set forth in claim 1, in which said micro-porous membrane is made ofpolyethylene, polypropylene or nylon.
 8. A separator for an alkali-zincbattery as set forth in claim 1, wherein said pores have a pore diameterof 0.05 through 0.125 microns.